Ceramic filter for a dust-containing gas and method for its production

Abstract

A ceramic filter for a dust-containing gas, which comprises a filter base having an average pore size of from 10 to 100 μm, with the pore size ratio at positions of 75 vol % and 25 vol % of the accumulated pore size distribution being at least 1.3, and a filter layer having an average pore size of from 0.2 to 10 μm fixed at least to the filtering side surface of the filter base so that the filter layer fills the pores open on the surface of the filter base.

What is claimed is: 
     
       1. A ceramic filter for a dust-containing gas, which comprises a filter base having an average pore size of from 10 to 100 μm, with the pore size ratio at positions of 75 vol% and 25 vol% of the accumulated pore size distribution being at least 1.3, and a filter layer having an average pore size of from 0.2 to 10 μm fixed at least to the filtering side surface of the filter base so that the filter layer substantially fills the pores open on the surface of the filter base. 
     
     
       2. The ceramic filter according to claim 1, wherein the pore size ratio at positions of 75 vol% and 25 vol% of the accumulated pore size distribution is at least 1.4. 
     
     
       3. The ceramic filter according to claim 2, wherein the filter base is made of cordierite. 
     
     
       4. The ceramic filter according to claim 3, wherein the filter layer is made of diatomaceous earth or rice hull ash. 
     
     
       5. The ceramic filter according to claim 2, wherein the filter base has a shape of a tube, and the filter layer is fixed to the inner surface of the tube. 
     
     
       6. The ceramic filter according to claim 1, wherein the filter base has an apparent porosity of from 35 to 50%. 
     
     
       7. The ceramic filter according to claim 1, wherein the filter base is made of cordierite. 
     
     
       8. The ceramic filter according to claim 7, wherein the filter layer is made of diatomaceous earth or rich hull ash. 
     
     
       9. The ceramic filter according to claim 1, wherein the filter base is made of silicon carbide. 
     
     
       10. The ceramic filter according to claim 1, wherein the filter base has a shape of a tube, and the filter layer is fixed to the inner surface of the tube. 
     
     
       11. A ceramic filter for a dust-containing gas, which comprises at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of at least 74 μm, and a matrix, wherein said matrix comprises silica (SiO 2 ) and alumina (Al 2  O 3 ) as the main components and contains from 0.05 to 1.0 wt%, based on the weight of the entire filter, of lithium oxide (Li 2  O). 
     
     
       12. The ceramic filter according to claim 11, wherein the matrix contains magnesium oxide (MgO). 
     
     
       13. The ceramic filter according to claim 11, wherein the cordierite grog is a cordierite grog crystallized from a glass having approximately the cordierite composition (2MgO.2Al 2  O 3 .5SiO 2 ). 
     
     
       14. The ceramic filter according to claim 13, wherein the filter material has an apparent porosity of at least 35%, and the bending strength at 800° C. of at least 75 kg/cm 2 . 
     
     
       15. The ceramic filter according to claim 13, wherein the matrix contains from 0.5 to 12 wt%, based on the weight of the entire filter, of TiO 2  and/or ZrO 2 . 
     
     
       16. The ceramic filter according to claim 11, wherein the cordierite grog is the one obtained by impregnating alumina sol, silica sol, titania sol, zirconia sol or a mixture of at least two types of these sols, to be cordierite grog, and after sintering the grog has an apparent porosity of at most 10%. 
     
     
       17. The ceramic filter according to claim 11, wherein the filter material has an apparent porosity of at least 35% and a bending strength at 800° C. of at least 75 kg/cm 2 . 
     
     
       18. The ceramic filter according to claim 16, wherein the matrix contains from 0.5 to 12 wt%, based on the weight of the entire filter, of TiO 2  and/or ZrO 2 . 
     
     
       19. A ceramic filter for a dust-containing gas, which comprises a filter base comprising at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of from 74 to 590 μm, and a matrix, wherein said matrix comprises silica (SiO 2 ) and alumina (Al 2  O 3 ) as the main components and contains from 0.2 to 0.6 wt%, based on the weight of the entire filter, of lithium oxide (Li 2  O), said filter base having an average pore size of from 10 to 100 μm, with the pore size ratio at positions of 75 vol% and 25 vol% of the accumulated pore size distribution being at least 1.3, and a filter layer having an average pore size of from 0.2 to 10 μm fixed at least to the filtering size surface of the filter base so that the filter layer fills the pores open on the surface of the filter base. 
     
     
       20. The ceramic filter according to claim 19, wherein the cordierite grog is a cordierite grog crystallized from a glass having approximately a cordierite composition (2MgO.2Al 2  O 3 .5SiO 2 ). 
     
     
       21. The ceramic filter according to claim 20, wherein the filter layer is made of diatomaceous earth or rice hull ash. 
     
     
       22. The ceramic filter according to claim 21, wherein the filter base has a shape of a tube, and the filter layer is fixed to the inner surface of the tube. 
     
     
       23. A method for producing a ceramic filter for a dust-containing gas, which comprises rubbing powder at least into the filtering side surface of a filter base having an average pore size of from 10 to 100 μm, with the pore size ratio at positions of 75 vol% and 25 vol% of the accumulated pore size distribution being at least 1.3, the powder to form a filter layer having an average particle diameter smaller than the average pore size of the filter base, and then fixing the powder with an inorganic binder. 
     
     
       24. The method for producing a ceramic filter according to claim 23, wherein a first powder is rubbed into the surface of the filter base, then a second powder having a particle size finer than the first powder is rubbed into the surface, then both said first and second powders are fixed to the surface of the filter base. 
     
     
       25. The method for producing a ceramic filter according to claim 24, wherein the average particle diameter of the first powder is from 20 to 70% of the average pore size of the filter base, and the average diameter of the second powder is from 2 to 15% of the average pore size of the filter base. 
     
     
       26. The method for producing a ceramic filter according to claim 24, wherein the molding of the filter is conducted by an isostatic press using a metal core mold and an outer cylindrical rubber mold, to form a tube and the powder is rubbed into the inner side surface of the filter base. 
     
     
       27. The method for producing a ceramic filter according to claim 23, wherein the inorganic binder is a solution containing silica as the main component, and this solution is sprayed to the surface into which said powder was rubbed, and then heated and dried for fixing. 
     
     
       28. The method for producing a ceramic filter according to claim 27, wherein the solution containing silica as the main component is a 1 to 5 wt% silica sol aqueous solution. 
     
     
       29. The method for producing a ceramic filter according to claim 23, wherein the molding of the filter is conducted by an isostatic press using a metal core mold and an outer cylindrical rubber mold, to form a tube and the powder is rubbed into the inner side surface of the filter base. 
     
     
       30. A method for producing a ceramic filter for a dust-containing gas, which comprises mixing at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of at least 74 μm and a powder material to form a matrix obtained by mixing a powder containing lithium oxide, clay and a cordierite powder having a particle size of less than 74 μm so that lithium oxide would be from 0.05 to 1.0 wt% based on the weight of the entire filter, adding an organic binder to the mixture, followed by molding and sintering. 
     
     
       31. The method for producing a ceramic filter according to claim 30, wherein the cordierite grog having an apparent porosity of at most 10% is prepared by mixing magnesia, alumina and siliceous sand to constitute approximately a cordierite composition (2MgO.2Al 2  O 3 .5SiO 2 ), melting the mixture and cooling it to obtain a glass, followed by heat treatment to crystalline the glass. 
     
     
       32. The method for producing a ceramic filter according to claim 31, wherein at the powder containing lithium oxide, a β-spodumene powder is added to the powder material to form the matrix. 
     
     
       33. The method for producing a ceramic filter according to claim 32, wherein a powder of ZrO 2  and/or TiO 2  is added to the powder material to form the matrix, in an amount of from 0.5 to 12 wt%, based on the weight of the entire filter. 
     
     
       34. The method for producing a ceramic filter according to claim 31, wherein the sintering is conducted at a temperature of from 1,310° to 1,380° C. 
     
     
       35. The method for producing a ceramic filter according to claim 30, wherein as the powder containing lithium oxide, a β-spodumene powder is added to the powder material to form the matrix. 
     
     
       36. The method for producing a ceramic filter according to claim 35, wherein a powder of ZrO 2  and/or TiO 2  is added to the powder material to form the matrix, in an amount of from 0.5 to 12 wt%, based on the weight of the entire filter. 
     
     
       37. The method for producing a ceramic filter according to claim 30, wherein a coke powder is mixed to the starting material, as a pore-forming material which forms pores when burned off. 
     
     
       38. The method for producing a ceramic filter according to claim 30, wherein the sintering is conducted at a temperature of from 1,310° to 1,380° C.